Pressure transducers used to measure the pressure of a process fluid (gas or liquid) flowing through a pipe are known in the art. Pressure transducers are used in highly sensitive environments including refineries, the aerospace industry and power plants. The accurate detection of flow conditions is critical in ensuring the operation and safety of the industries in which pressure transducers are used.
Many pressure transducers utilize a semiconductor sensor comprised of a parallel plate capacitor. Parallel plate capacitors are known in the art wherein one plate is formed of a semiconductor material and the second plate is formed of a metal deposited on a dielectric. A second or spacing dielectric (e.g., air or silicon oil) fills the space between the two capacitor plates.
Under normal operations, transducers are subject to physical forces (including vibrations, compression and stress), pressure irregularities and temperature fluctuations which can decrease the accuracy of the measurements. Variations in temperature are extremely common and are particularly troublesome. Temperature changes cause the separation between the capacitor plates to vary due to thermal expansion or contraction of the semiconductor and the metalized dielectric. Temperature also affects the dielectric constant of the spacing dielectric (i.e., the silicon oil). The surface area of the plates also vary with the change in temperatures. The separation distance between the plates, the change in dielectric constant of the spacing dielectric and the variation in plate surface area each effect the capacitance of the sensor and ultimately decrease the accuracy of any pressure readings.
Prior art methods of compensating for temperature utilize a separate sensor (e.g., a thermistor which comprises two dissimilar metal bonded together--each metal having different coefficients of expansion) to provide temperature data. A microprocessor circuit then corrects for temperature variations through a series of calculations. Alternatively, the various components of the pressure transducers are "matched" in an attempt to make all components expand and contract the exact amounts with the fluctuations in temperature, thereby eliminating temperature as a variable.
Traditional pressure sensors are designed to measure either a differential or an absolute pressure. They are usually optimized to accurately measure only a relatively small range of pressures. For instance, a sensor that is designed to accurately measure an absolute pressure that varies from 0 to 50 psi gage, will be unable to measure any pressures over 50 psi. On the other hand, a sensor that is designed to measure pressure in the range of 0 to 5000 psi gage cannot accurately measure a pressure that only varies from 0 to 50 psi gage. If an accurate measurement of this lower pressure range is desired, a separate sensor will be required. Moreover, if measurements of differential pressure or gage pressure are also required, additional sensors would be needed.